1. Field of the Invention
The invention relates to the field of fuel assemblies for nuclear reactors, and, in particular, concerns a replacement rod for replacing a damaged fuel rod in a fuel assembly, and also concerns a method of reducing the peaking factor in a nuclear reactor fuel assembly in which one or more fuel rods are removed.
2. Description of the Prior Art
The core of a pressurized water nuclear reactor used for power generation typically includes a plurality of fuel assemblies, each fuel assembly typically comprising a parallel array of elongated, cylindrically-shaped fuel rods, a central instrument thimble, and several guide tubes dispersed among the fuel rods for insertion of control rods. Circulating water in a primary cooling system passes through the core between the rods and the guide tubes to cool the core and to transport heat to steam generators. Each fuel rod typically includes a stack of fissile fuel pellets, such as, for example, UO.sub.2 pellets isotopically enriched with U235 to about 5% by weight, contained within a thin cladding of a zirconium alloy, such as Zircalloy-4.
The cladding is highly resistive to damage from radiation, heat, and chemicals in the cooling water, however, pinholes and cracks develop in some fuel rods after months or years in service. The damaged fuel rods must be removed from the core to prevent leakage of radioactive fuel and fission by-products from the fuel rod into the primary cooling system. Typically, the damaged fuel rods are replaced with a non-fissile rod of the same dimensions as the removed fuel rod, fabricated of a material commonly used in nuclear reactor structures, such as stainless steel or a Zirconium alloy, like Zircalloy, or alternatively the damaged fuel rods are replaced with a fuel rod containing UO.sub.2 pellets having a low U235 isotopic enrichment, such as a natural enrichment or depleted uranium.
The power output of fuel rods located nearby a prior art replacement rod typically rises because the replacement rod does not absorb neutrons to the same extent as the damaged fuel rod that it replaced. This is true for nonfissile stainless steel or Zircalloy replacement rods and for low-enrichment uranium rods. The neutrons not absorbed by the replacement rod, which otherwise would have been absorbed by the damaged fuel rod, can be absorbed by the nearby fuel rods, causing a higher fission rate and a faster burn-up of fuel in the nearby fuel rods. The peaking factor, which is defined as the ratio of the power output of the fuel rod with the highest power output to the average power output of all the fuel rods in a fuel assembly, is therefore typically raised by the replacement of a damaged fuel rod with a replacement rod. In some cases this increase of the peaking factor cannot be accomodated by the available margins, and relocation or replacement of the entire assembly may be required. Such action reduces the neutronic and economic efficiency of the reactor. Therefore, it is desirable to provide a replacement rod for damaged fuel rods that does not increase the peaking factor in a fuel assembly to the extent caused by prior art replacement rods.